1. Field of the Invention
The present invention relates to a digital transmission system, and more particularly to a processing method and apparatus for a Network Node Interface (NNI) for Synchronous Digital Hierarchy (SDH) specified by the recommendations of the International Telegraph and Telephone Consultative Committee (CCITT).
2. Description of the Related Art
The Recommendations G702, G707, G708, G709 of CCITT specify a multiplexing method and its details for a Synchronous Digital Hierarchy (SDH) and Network Node Interface (NNI) for multiplexing various digital communications networks from bit rates of 1544 kbit/s, 2048 kbits/s, 6312 kbit/s, 34368 kbit/s, 44736 kbit/s, and 139264 kbits/s to bit rates of 155.52 Mbit/s or higher (CCITT BLUE BOOK Recommendations G707, G708, G709, 1989, revised 1990).
The SDH is composed of hierarchical elements which are arranged from lower to higher levels, as follows:
Container (C-n, n=1-4) which is composed of information payload comprising a synchronizing information signal of 1.5 Mbit/s through 139 Mbit/S;
Virtual Container (VC-n) of lower order (n=1, 2) or higher order (n=3, 4) that adds a Path Overhead (POH) as control information to the Container (C-n) and is mapped in one block frame;
Tributary Unit (TU-n, n=1, 2, 3) which includes the lower order VC in a floating manner and is aligned by the addition of a pointer (TU pointer) indicating the offset of the payload frame start;
Tributary Unit Group (TUG-2, 3, 4) in which plural TU-1, 2 or one TU-3 multiplexed in higher order VC-3, 4 is mapped in fixed location;
Administrative Unit (AU-n, n=3, 4) which comprises information payload composed of higher order VC-3 or VC-4 with plural TUGs and higher order C-3, 4 multiplexed therein, and a pointer (AU pointer) indicating the offset of the payload frame start relatively to the multiplex section frame start; and
Synchronous Transport Module (STM-n, n=1, 4, 16) which is composed of an AU group (AUG) as information payload with one or more AU-n multiplexed therein and is mapped in one block frame of 9 rows and 261.times.n columns with the addition of Section Overhead (SOH), the STM being repeated every 125 microseconds.
A basic STM, which is termed STM-1, is composed of 9 rows by 270 columns and defined at 155520 kbit/s.
In the whole process of multiplexing and transmitting each Container as STM-1 and separating received STM-1 into each Container, each NNI must renew the AU pointer to the TU pointer when a cross-connection is to be made with respect to each VC, but this process is not defined in the Recommendations.
Heretofore, such pointer renewal has been carried out as follows: All STM-1 signals having respective AU pointers are demultiplexed into signals each including lower order VC, i.e., the signals are rearranged with VCs as channels.
Thereafter, each respective offset, i.e., each TU pointer, is added to the VC channels, and they are multiplexed into TU-n signals providing higher order VC-3 or VC-4. Each AU pointer is then added to produce AU signals. Finally, SOH is added to the AU signals to provide the STM-1 signal.
Since the demultiplexing and multiplexing processes for the cross-connection are required for all levels from the lowest to the highest levels, conventional pointer renewal has been time-consuming, and the delay time caused by pointer renewal has been long.